Unhook one battery cable
Test battery voltage, 12.3 to 12.8v is expected, write it down
1 year old battery 12.6v at least
Wait 3 or 4 hours and test it again, the longer the better, should be EXACTLY the same with cable unhooked
If its dropping then replace battery under warranty, it has an internal short, self draining, for sure
Above 12.3v the engine/starter motor should start easily, under 12.3v is when there are issues starting
If battery voltage is above 12.2v and engine/starter motor don't work but jumping does, then battery cable terminal is bad, positive or negative BOTH need to pass the same AMPs for a DC system to work
i.e. jumper cables are usually clamped to the outside of the battery terminal cables bypassing connection to battery, external and internal
So could be cable ends need to be cleaned/changed or internal connection inside battery is bad
After engine is started NO POWER comes from the battery, battery is not used at all when alternator is generating voltage
Max battery volts is 12.9/13.0v, brand new out of the box, lol
Alternator generates 13.5-14.8volts
So voltage flows TO the battery, to keep it charged up, so battery is off line with engine running
Battery's only purpose is to start the engine